Laminar gamma ferric oxide



Nov, 10, 1970 P. Y. HWANG 3,539,294

LAMINAR GAMMA FERRIC OXIDE Filed July 5, 1968 HIGH PRODUCT PRESSURE ToFLUID COLLECTOR LOW PRESSURE HOT AIR k FROM HEATER INVFNTOR. PAUL Y.HWANG BY flywea ATTORNEY United States Patent 3,539,294 LAMINAR GAMMAFERRIC OXIDE Paul Y. Hwang, Palo Alto, Calif., assignor to AmpexCorporation, Redwood City, Calif., a corporation of California FiledJuly 5, 1968, Ser. No. 742,772 Int. Cl. C01g 49/02; B013 2/00; B02c19/06 U.S. Cl. 23200 3 Claims ABSTRACT OF THE DISCLOSURE Gamma ferricoxide having high acicularity is produced by subjecting a slurry ofyellow iron oxide to a jet fluid energy mill which serves to break upthe particles along their long axes, thus improving the acicularity ofthe particles. The particles can then be converted to gamma ferric oxidein the usual manner, and the acicularity carries over to the gamma form.Such particles of acicular gamma ferric oxide produce magnetic recordingmedia of improved performance.

BACKGROUND OF THE INVENTION Field of the invention Acicular gamma ferricoxide.

Description of the prior art Previously, expensive chemical conversionprocesses were necessary to produce yellow iron oxide, and ultimatelygamma ferric oxide, of high acicularity. The present invention providesa simple mechanical method of converting the inexpensive yellow ironoxide of commerce to an acicular form.

SUMMARY OF THE INVENTION It has been long recognized in the manufactureof magnetic recording media that the acicularity of the magneticparticles, such as gamma ferric oxide, is highly important both from thestandpoint of output and noise, particularly at short wavelengths.

Gamma ferric oxide particles inherit their shape from the precursoralpha ferric oxide monohydrate and heretofore it has been necessary toemploy expensive chemical conversion steps to secure alpha ferric oxidemonohydrate having the desired acicularity. Alpha ferric oxidemonohydrate is available commercially at low cost as yellow iron oxide,a pigment utilized in the paint industry.

The present invention provides an inexpensive, purely mechanicalbreaking up of the particles in such a manner that the acicularity ofthe particles is greatly improved. When the alpha ferric oxidemonohydrate is then converted to gamma ferric oxide, the acicularity isretained and magnetic recording media of improved characteristics can beproduced therefrom.

Accordingly, it is an object of the present invention to provide aninexpensive conversion means for improving the acicularity of the commonyellow iron oxide of commerce.

Another object of the present invention is to produce gamma ferric oxidehaving a high acicularity ratio.

A further object of the invention is to provide a method of makingmagnetic recording media of improved output and signal to noise ratio,particularly at short wavelengths.

Other objects will be apparent from the balance of the specification.

BRIEF DESCRIPTION OF THE DRAWING The sole figure is a side view of anapparatus for carrying out the present invention.

3,539,294 Patented Nov. 10, 1970 DESCRIPTION OF THE PREFERREDEMBODIMENTS In carrying out the present invention a fluid energy mill isemployed. Such mills are described in the text Unit Operations ofChemical Engineering (McGraw-Hill, 1956), page 233. A typical mill isshown in the drawing. Such a mill comprises a torus 1, arranged in agenerally vertical configuration having an ascending section 3 and adescending section 5. An opening 7 is provided for the introduction of ahigh pressure fluid tangentially to the flow of material within thetorus while a screw feeder 9 is provided immediately below the opening 7for the introduction of the wet feed. It is clear from the drawing thatas the material in wet form is introduced into the mill, the jet of highpressure fluid from the opening 7 impinges upon it. Near the bottom ofthe mill there are a plurality of relatively large openings 11 leadingto a source of low pressure hot air 13. Near the top of the descendingcolumn 5 is provided an outlet 15. Exhaust air and product are takenfrom this outlet 15. Because of the centrifugal action of thecirculating fluid with the torus, relatively large particles will berecirculated for further fracture while relatively small particles willbe removed through the Opening 15. Eventually, of course, largeparticles will be fractured by the attrition produced within the mil andwill eventually be removed as small particles through the opening 15.

In operation, a slurry is pumped into the drier and is dispersed anddeagglomerated by the stream of hot pressure fluid which can be eithersuperheated steam or compressed air. It is believed that the laminarfracture of the yellow oxide particles occurs at this moment. Thedispersed particles are instantly dried by a current of hot air fedthrough the openings 11. It is obvious that in order to dry the slurry,an adequate volume of hot gas at a reasonably high temperature should beemployed. Low mill temperatures require a large volume of air and highmill temperatures make for a poor utilization of heat. However, thevolume and temperature should be adjusted so that drying issubstantially instantaneous and that no wet particles move into theascending column 3.

It has been found that the mill pressure is an important factor. Highmill pressure will bring about a violent agitation of the particleswithin the grinding chamber wherein the particles may break along theirshort axes to produce particles of poor acicularity. On the other hand,a positive pressure must be maintained within the mill to assureadequate circulation and product removal. Thus the pressure within themill must be maintained within the range of 0.1 to 10 p.s.i.g. andpreferably from 0.5 to 2 p.s.i.g.

Similarly, since the laminar breakage occurs during the dispersion ofthe slurry with the high pressure fluid, it is obviously necessary tomaintain a minimum pressure in line 7 to cause a proper fracture. Inpractice it has been found that this pressure must be at least 50p.s.i.g.

It has been found essential for conducting the process of the presentinvention to feed the material into the mill in the form of a slurry.When it was attempted to repeat the process of the examples with a drypowder, no increase in acicularity could be found. At low pressures withdry feed there is no change in particle size and as the pressure goesup, the particles break along their short axes, defeating the purposesof the present invention.

In each of the following examples, the starting material was ordinaryyellow iron oxide of commerce as is used by the paint industry. This isalpha ferric oxide monohydrate and the particles had an averageacicularity, i.e., the ratio of the length to breadth, of 4.7 to 1. Theparticles had an average Stokes equivalent diameter of 0.55 micron.

3 EXAMPLE I Seven hundred pounds of the yellow oxide described above wasadded to thirteen hundred pounds of water and the mixture washomogenized in a mixer to obtain a smooth slurry. The slurry was fedinto an 8 inch diameter jet fluid energy mill, manufactured by the FluidEnergy Processing and Equipment Company at a rate of 600 pounds perhour. High pressure steam at 150 p.s.i.g. fed at a rate of 850 poundsper hour was used to atomize the slurry. Action of the mill was suchthat laminar fractures were produced in the particles, i.e., theparticles were broken on their long axes to produce long slenderparticles. The slurry was dried in a current of hot air at EXAMPLE III Asample of the original yellow oxide previously described was directlyconverted to gamma ferric oxide and made into a magnetic tape asdescribed in Example I.

Three tapes were tested in an Ampex VR2000 video recorder at inches persecond. The following results were obtained:

Output, db Noise, db

Treated Treated Treated Treated oxide oxide oxide oxide Untreated tapetape Untreated tape tape Frequency oxide tape No. 1 No. 2 oxide tape No.1 No. 2

590 F. at thirteen hundred s.c.f.m. while maintaining the temperature ofthe mill between 240 and 280 F. The mill pressure was maintained below 2p.s.i.g. The dried product had an average Stokes equivalent diameter of0.41 micron and an acicularity ratio of 6. The product was thendehydrated to alpha ferric oxide at 350 C., reduced to magnetite at 350C. in an atmosphere of hydrogen; it was then oxidized to gamma ferricoxide in air at a temperature of about 250 C.

A formulation was then made up of the following (all parts are byweight):

This mixture was ground in a pebble mill for 72 hours and the followingwas added:

Parts Polyurethane resin 190 Polyvinylidiene chloride resin 169Methylisobutyl ketone 244 Methyl ethyl ketone 1194 Grinding wascontinued for another 24 hours to produce a well dispersed slurry.

The slurry was coated on a 1 mil thickness polyester film to produce adried coating having a thickness of 0.4 mil. The particles were orientedto the longitudinal direction with a thousand gauss magnet. The finishedfilm was tested as a magnetic tape as is hereinafter described.

EXAMPLE [I A second sample of the yellow oxide was treated as isdescribed in Example I except that the steam pressure was The jet millproduces an unusual type of grinding action. A high pressure fluid isfed into the grinding chamber tangentially, causing violent agitation ofa slurry of particles wtihin the grinding chamber and causing theirimmediate fracture. This produces the unique laminar fracture desiredresulting in the increase in acicularity. When yellow oxide was groundin an ordinary attrition mill, no increase in acicularity was found.

I claim:

1. The process of making acicular alpha ferric oxide monohydratecomprising forming an aqueous slurry of alpha ferric oxide monohydrateand subjecting the slurry to the action of a fluid energy mill operatedat an internal pressure of from 0.1 to 10.0 p.s.i.g. and at a gas feedpressure of at least 50 p.s.i.g. and drying the particles within saidmill to recover alpha ferric oxide monohydrate having improvedacicularity.

2. The process of claim 1 wherein the mill is operated at a pressure offrom 0.5 to 2.0 p.s.i.g.

3. The process of claim 1 wherein the resulting alpha ferric oxidemonohydrate is dehydrated to alpha ferric oxide, reduced to magnetite byheating the same with a reducing gas and then oxidized to gamma ferricoxide wherein the gamma ferric oxide preserves the increased acicularityof the treated alpha ferric oxide monohydrate.

3rd ed.,

U.S. Cl. X.R. 23293; 241-5

